Synthesis of β-Hydroxy α-Amino Acids Through Brønsted Base-Catalyzed syn-Selective Direct Aldol Reaction of Schiff Bases of Glycine o-Nitroanilide

Here we report the highly enantio- and syn-selective synthesis of β-hydroxy α-amino acids from glycine imine derivatives under Brønsted base (BB) catalysis. The key of this approach is the use of benzophenone-derived imine of glycine o-nitroanilide as a pronucleophile, where the o-nitroanilide framework provides an efficient hydrogen-bonding platform that accounts for nucleophile reactivity and diastereoselectivity.

D ue to their prevalence in natural products, including antibiotics and enzyme inhibitors and their presence as structural components of many biologically active products, βhydroxy α-amino acids are compounds of high interest in medicinal chemistry. 1 Thus, different approaches for their enantioselective synthesis have been reported, most of them relying on the use of glycine derivatives. 2 Among these and since their introduction by O'Donnel and Eckrich in 1978, 3 glycine Schiff bases stand up as the most appealing substrates due to their bench stability. In this context, the asymmetric aldol reaction of glycine Schiff bases is very effective for the production of β-hydroxy α-amino acids because, concomitant to the assembly of the 1,2-aminoalcohol functionality during the carbon−carbon bond forming step, up to two vicinal stereogenic centers are created in a single synthetic operation. In 1991, Miller and Gasparski reported the first catalytic direct aldol reaction of benzophenone imines of glycine esters using phase-transfer catalysis. The method leads to anti-adducts but in low diastereoselectivities and negligible enantiomeric excesses. 4 Following this report, other protocols involving phase-transfer catalytic conditions, metal catalysis, or the use of lithium/(−)-sparteine have also been described. 5−7 Among these, the only report concerning the enantioselective direct synthesis of syn-isomers from glycine Schiff bases has been documented, as far as we know, by Trost using a zinc-ProPhenol catalyst. 8 The reaction works well for α-substituted aldehydes but provides less satisfactory enantioselectivities for linear alkyl aldehydes.
Herein we report the first aldol reaction of Schiff bases of glycine derivatives by Brønsted base (BB) catalysis, which provides syn β-hydroxy α-amino acids in high diastereo-and enantioselectivity. The key for this development is the use of a glycine o-nitro anilide derivative in combination with an ureidopeptide-based BB catalyst.
Benzophenone imines of glycine esters have shown to be very efficient substrates with applications in many transformations. 3, 9 However, their use in enantioselective synthesis has been mainly limited to metal 10 and phase transfer 11 catalysis, while their development in organocatalysis 12 remains essentially underexplored. 13 The main reason that can account for this deficiency is the relatively low acidity of the methylenic carbon, which precludes enolate generation through deprotonation by the weak BB catalysts 14 usually employed. Only recently, three examples have been documented (Figure 1a) in which this problem has been solved by using more acidic structural analogues as fluorenone imine 15 (A), 2-hydroxybenzophenone imine 16 (B), and (R)-3-hydroxy-[1,1′-binaphthalene]-2-carbaldehyde 17 imine (C) of glycine derivatives. The increased acidity of these iminoesters is the result of structural modifications on the imine function by the incorporation of motifs that promote either stabilization of the corresponding conjugate base by extensive charge delocalization (A) or intramolecular hydrogen bonding as in B and C. It was our consideration that the installation of an onitroaniline motif in the carboxy terminus might be another possibility to increase α-carbon acidity of glycine Schiff bases (Figure 1b). It has been reported that o-nitroanilides of simple carboxylic acids exhibit intramolecular hydrogen bonding between the oxygen of the nitro group and the hydrogen of the amide moiety, 18 largely facilitating hydrolysis by enzymes. 19 We expected that Schiff base 1, besides this Hbond pattern, should exhibit an additional H-bonding with the imine nitrogen increasing the acidity of the methylenic carbon. If this were the case, enolization of 1 by a weak tertiary amine base should be feasible while generating an E-enolate ion pair, mainly.
The first promising piece of evidence supporting this assumption was provided by a single-crystal X-ray analysis of compound 1, which revealed hydrogen bond lengths of 1.987 Å and 2.149 Å that fit well with the proposed bifurcated hydrogen bond motif 20 (Figure 2). Interestingly, the X-ray analysis also showed an additional hydrogen bonding (2.234 Å) between the o-aromatic hydrogen and the carbonyl oxygen. Therefore, while amides are known to be reluctant to enolization, 21 we expected these structural features should render substrate 1 quite promising for Brønsted base-promoted stereoselective transformations.
Initially, our approach was evaluated from the reaction of benzophenone imine 1 22 with hydrocinnamaldehyde 4a (Scheme 1) mediated by squaramides C1 and C2. 23 Using these bases, the reaction indeed proceeded to give the aldol product 5a after one-pot reductive workup, but with very poor diastereoselectivity and negligible enantioselectivity for the major syn isomer, albeit good for the minor isomer (Table 1, entries 1 and 2). Using the parent ureas, C3 and C4, much better diastereocontrol was achieved (entries 3 and 4), but the enantioselectivity of product 5a was still poor. To improve stereocontrol through the incorporation of additional H-bond donors 24 during the reaction, we focused on ureidopeptidederived Brønsted bases previously developed by us. 25 It was gratifying to observe that the new variants C5, C6, and C7 provided 5a with diastereomeric ratios greater than 98:2 and in each case with good enantioselectivity (entries 5, 6, and 8).   Further improvement was achieved using catalyst C6 and carrying out the reaction at 0°C, and 5a was obtained in 77% isolated yield and 94% ee (entry 7). Although in these reactions, the formation of small amounts (10−15%) of the cycled product 8a was also observed, 5c its reductive workup also furnished the amino alcohol derivative 5a. 22 Likewise, iminoamides 2 and 3 upon treatment with 4a in CH 2 Cl 2 at 0°C led to products 6a and 7a in less than 24 h with good isolated yields and stereoselectivities, Table 1. The best result was attained for the latter, which was obtained essentially as a single syn-diastereomer and with excellent enantioselectivity. 26,27 The scope of the new glycine amide reagent 3 to the synthesis of syn-β-hydroxy α-amino acids was next examined from a selection of representative enolizable aldehydes (Table  2). With the exception of α-branched aldehydes such as isobutyraldehyde and cyclohexanecarboxaldehyde, which were inert to this system, results were consistently good. As the data in Table 2 show, syn-β-hydroxy α-amino acids bearing short and long linear chains, which cannot be accessed by the above known methods, vide supra, may be prepared with very good yields and ee values. Isovaleraldehyde and aldehydes bearing side chains with functional groups (e.g., ester, carbamate, and ether) are equally tolerated to give the respective syn-isomer with good enantiomeric excess. Importantly, in every case under these reaction conditions, self-aldol products from the corresponding enolizable aldehyde 4 were not detected. 28 Likewise, product 7a was chemically and stereochemically unchanged when exposed to treatment with aldehyde 4b at room temperature in the presence of both Et 3 N (20 mol %) and Schreiber achiral diarylthiourea 23 (20 mol %) for 24 h, thus indicating product stability.
The absolute configuration of the adducts was established by X-ray analysis of compound 10 derived from the reaction of 3 with hydrocinnamaldehyde, Scheme 2, and by assuming a uniform reaction mechanism. 29 On the other hand, treatment of aldol product 7a with 2 equiv of (Boc) 2 O led to 11, which upon Evans hydrolytic conditions provided the carboxylic acid 12 along with N-Boc nitroaniline. 30 The above experimental results clearly show that benzophenone imines of glycine o-nitroanilides are acidic enough to react in BB-catalyzed reactions under soft enolization conditions. To further prove the significance of the intramolecular hydrogen bonding in o-nitroanilides 1−3, 31 benzophenone imines 13, 14, and 15 were prepared and subjected to treatment with 4a under the above conditions (Scheme 3), and in no case was a reaction observed. Likewise, in an attempt to strengthen the hydrogen bonding, compound 16 with an additional nitro group in the para position of the aromatic ring was also prepared. While, in this case, the reaction proceeded, product 17 was obtained in a modest yield and in an almost racemic form.
In order to understand the origin of the syn selectivity in the reaction, we performed an initial evaluation of the relative stabilities of the naked enolates derived from ketimine 3, 32 and, as expected, it was found that the naked E-enolate was more stable than the respective Z-enolate, ΔG E/Z = +3.4, 33 ( Figure  3a). Our interpretation of the higher stability of the E enolate relies on three main pieces of evidence. First, the Pauli repulsion of the negative charge of the amide oxygen atom and the lone pair of the imine nitrogen present in the Z-enolate. Second, the existence of an additional H-bonding interaction between the lone pair of the iminic nitrogen and the NH moiety only present in the E-enolate. Lastly, the planarity observed in the E-enolate (dihedral θ 1234 close to 180°) that points to the existence of extended π-conjugation that is not present in the Z counterpart (θ 1234 far from 180°or 0°). Remarkably, the internal NH···O 2 N and CH···O − hydrogen− bonding interactions shown in the X-ray analysis are also detected in this study. Assuming that the ketimine E-enolate (E-Ar) participates during the reaction, a plausible explanation of the observed syn-selectivity can be provided by the model shown in Figure 3b, wherein the si face of the E-enolate approaches the si face of the aldehyde.
While these results confirm the initial assumption concerning the role of the 2-nitroanilide tether, further evidence is provided from the reactions in Scheme 4. For instance, treatment of the pyridylacetic acid-derived 2-nitroanilide 18 with 20 mol % of TEA and hydrocinnamaldehyde for 48 h at rt, followed by in situ reaction with Ac 2 O and pyridine, provided the corresponding aldols 20 with >99% conversion (67% combined isolated yield) and as a 50:50 diastereomeric mixture. However, when the same reaction conditions were used with the 4-nitroanilide 19 and the anilide derivative 20, compounds 23 and 24 were produced in lower conversions (50% and 33% respectively). Similarly, the essential absence of reactivity of 21, which bears the 2-nitroanilide motif but lacks the heteroatom at the aromatic ring, for producing compound 25, provides further proof of the significance of the whole Hbonding network in the starting pronucleophile for reactivity.
In summary, an effective organocatalytic direct access to synβ-hydroxy α-amino acids is reported. The strategy is based on Schiff bases of glycine o-nitroanilide, wherein the o-nitroanilide motif is key for enolate generation by a soft Brønsted base allowing direct reaction with aldehydes under efficient diastereo-and enantiocontrol. Further applications of both the nitroanilide tether and the catalyst and/or variants may be easily anticipated.
■ EXPERIMENTAL SECTION General Information. All nonaqueous reactions were performed under an inert atmosphere using oven-dried glassware, and the mixtures were magnetically stirred. Yields refer to chromatographically purified and spectroscopically pure compounds, unless otherwise stated. Heat requiring reactions were performed using a hot plate with a sand or an oil bath and a condenser. Reactions requiring low temperatures were performed using cooling bath circulators, Huber T100E, and acetone or isopropanol baths. Organic layers washed with aqueous phases were dried over MgSO 4 or Na 2 SO 4 and filtered through cotton. Organic solvents were evaporated under reduced pressure using rotavapors Buchi R-100, R-200, and R-210, the latter equipped with a Buchi V-700 vacuum pump and a Buchi V-850 vacuum controller, appropriate for the evaporation of solvents when products were volatile compounds. For the complete removal of solvents, a vacuum pump, Telstar Top-3 (≈ 0.5 mmHg), was employed. Reagents were purchased from different commercial suppliers (Aldrich, Across, Alfa Aesar, Fluka, TCI, Merck, Fluorochem, etc.), stored as specified by the manufacturer, and used without previous purification unless otherwise stated. Triethylamine was purified by distillation. When anhydrous solvents were required, they were dried following established procedures. 34 Dichloromethane was dried over CaH 2 , and tetrahydrofuran was dried by filtration through activated alumina (powder 150 mesh, pore size 58 Å, basic Sigma-Aldrich) columns. Reactions and flash chromatographic columns were monitored by thin-layer chromatography (TLC) using Merck silica gel 60 F254 plates and visualized by fluorescence quenching under UV light, Fisher Biolock lamp VL-4LC, λ = 254 and 365 nm. In addition, TLC plates were stained with a dipping solution of potassium permanganate (1g) in 100 mL of water (limited lifetime), followed by heating and charring with 1% w/w ninhydrin in ethanol followed by heating. Chromatographic purification was performed on Merck ROCC 60 silica gel 40−63 μm as stationary phase and a suitable mixture of solvents (typically hexane: ethyl acetate or dichloromethane/methanol) as eluent.  melting point apparatus and were uncorrected. NMR spectra were recorded using a Bruker Avance 300 (300 MHz for 1 H, 75 MHz for 13 C) spectrometer, Bruker 400 spectrometer (400 MHz for 1 H, 100 MHz for 13 C), or Bruker AV-500 spectrometer (500 MHz for 1 H, 125 MHz for 13 C). Chemical shifts (δ) are quoted in parts per million referenced to the residual solvent peak, usually CDCl 3 , 1 H (δ = 7.26) and 13 C (δ = 77.0). The multiplicity of each signal is designated using the following abbreviations: s, singlet; d, doublet; t, triplet; q, quartet; m, multiplet; brs, broad singlet. Coupling constants (J) are reported in hertz (Hz). The MestrReNova Mnova 8.1 program was used to process and edit the registered spectra. Mass spectra were recorded on an ESI-ion trap mass spectrometer (Agilent 1100 series LC/MSD, SL model) on a UPLC-DAD-QTOF, ultra high-performance liquid chromatography−mass spectrometer, Waters UPLC ACQUITY, Waters PDA detector, Waters Sunapt G2, or on an Agilent Thermoquest LCT spectrometer. Mass spectrometry analyses were performed in the General Research Service (SGIker) of the University of the Basque Country (UPV/EHU). Enantiomeric excesses were determined using analytical high-performance liquid chromatography (HPLC) performed on Waters 600-E (equipped with 2996 and 2998 photodiode array UV detector) employing Daicel columns (IA, IF) and Phenomenex Lux (cellulose 3 μm, amylose 3 μm). The X-ray diffraction analysis experiments were conducted in the General Research Service (SGIker) of the University of the Basque Country (UPV/EHU) using difractometers for monocrystals. Aliphatic aldehydes 4a, 4b, 4c, 4d, 4e, and 4f are commercially available and were purchased from commercial suppliers. Aldehydes 4g, 4h, 4i, and 4j were prepared following literature procedures. 35 All aldehydes were dissolved in Cl 2 CH, treated with an aqueous saturated solution of NaHCO 3 , and subsequently distilled before their use in the aldol reaction.
Synthesis of Catalysts. Bifunctional organocatalysts C1, 36 C2, 37 C3, 38 and C4 39 were prepared following reported procedures. Catalysts C5, C6, and C7 were synthesized following a modification of the procedures previously reported. 25a Synthesis of Catalysts C5, C6, and C7. a) Preparation of N-Protected L-tert-Leucine. 40 In the first step, pyridine (0.9 mL, 11 mmol, 1.1 equiv) was added to a stirred solution of p-nitrophenyl chloroformate (2.2 g, 11 mmol, 1.1 equiv) in dichloromethane (13.6 mL). The white slurry was cooled to 0°C, and the corresponding alcohol (10 mmol, 1 equiv) was slowly added in at the same temperature. After addition, the mixture was allowed to warm to room temperature and stirred for 16 h. The reaction mixture was diluted with CH 2 Cl 2 (40 mL) and washed with 1 M HCl (20 mL), water (20 mL), and brine (20 mL). The organic layer was dried over MgSO 4 and concentrated under reduced pressure. The residue was used in the next step without further purification.
In the second step, to a stirred solution of L-tert-leucine (1.31 g, 10 mmol, 1 equiv) in 10% Na 2 CO 3 (26 mL) and dimethylformamide (10 mL) was added a solution of the corresponding carbonate (10 mmol, 1 equiv) in dimethylformamide (30 mL) slowly at 0°C. The mixture was stirred at the same temperature for 1 h and at room temperature for 16 h. The reaction mixture was poured into H 2 O (100 mL) and washed with Et 2 O (3 × 50 mL). The aqueous layer was cooled in an ice bath and acidified with concentrated HCl, followed by extraction with EtOAc (3 × 50 mL). The combined organic phases were washed with brine (5 × 50 mL), dried over MgSO 4 , and concentrated under reduced pressure.
(S)-2-(((Anthracen-9-ylmethoxy)carbonyl)amino)-3,3-dimethylbutanoic Acid. 25a The title compound was prepared from 9-anthracenemethanol (2.08 g, 10 mmol) following the general procedure. Purification by flash column chromatography on silica gel (hexane/EtOAc, 70:30) afforded the title compound as a white solid. Yield: 88% (3.2 g, 8.8 mmol). All of the spectroscopic data were coincident with those previously reported. 1 13  (S)-3,3-Dimethyl-2-(((naphthalen-2-ylmethoxy)carbonyl)amino)butanoic Acid. 25a The title compound was prepared from 2-naphthalenemethanol (1.58 g, 10 mmol) following the general procedure. Removal of the remaining phenol was not possible by column chromatography. Therefore, after the work-up described in the general procedure, the crude was dissolved in Et 2 O (30 mL) and basified with NaHCO 3 saturated aq. solution. The aqueous phase was washed with Et 2 O (3 × 20 mL), acidified with concentrated HCl, and extracted with EtOAc (3 × 25 mL). The combined organic phases were dried over MgSO 4 and evaporated under reduced pressure to afford the title compound as a white solid. Yield 48% (1.5 g, 4.8 mmol). All of the spectroscopic data were coincident with those previously reported. 1  To a cooled solution of the corresponding N-protected α-amino acid (5 mmol, 1 equiv) in dry THF (20 mL) were added isobutyl chloroformate (0.65 mL, 5 mmol, 1 equiv) and N-methylmorpholine (0.6 mL, 5 mmol, 1 equiv) at −20°C. The mixture was stirred at the The Journal of Organic Chemistry pubs.acs.org/joc Note same temperature for 20 min. Then, a suspension of NaN 3 (0.48 g, 7.5 mmol, 1.5 equiv) in 5 mL of H 2 O was added, and the reaction mixture was stirred at the same temperature for 30 min. The organic layer was separated and evaporated, and the residue was dissolved in CH 2 Cl 2 (30 mL) and washed with water (15 mL). The organic phase was dried over MgSO 4 , filtered, and concentrated in vacuo to give a yellow oil, which was dissolved in dry CH 2 Cl 2 (10 mL). The resulting solution was stirred at 40°C under nitrogen for 1−2 h. The reaction was monitored by IR analysis until the disappearance of the azide band (from azide ≈2136 cm −1 to isocyanate ≈2239 cm −1 ). After isocyanate generation, (1S,2S)-2-(piperidin-1-yl)cyclohexan-1amine 41 was added (638 mg, 3.5 mmol, 0.7 equiv), and the reaction mixture was stirred for 16 h at room temperature. The solvent was evaporated under reduced pressure, and the residue was purified by flash column chromatography on silica gel to afford the desired catalysts.
[α] D 23 −15.6 (c 1, CH 2 Cl 2 ). 1 H NMR (500 MHz, DMSO-d 6  Preparation of Ketimine Hydrochlorides. The imine hydrochlorides employed for the study were prepared by adapting literature procedures. 42 Dry diethyl ether (50 mL) was added to a three-necked round-bottom flask equipped with a reflux condenser containing magnesium powder (434 mg, 20 mmol, 1 equiv) and iodine (20 mg). The resulting suspension was heated to mild reflux, and the corresponding bromobenzene was added dropwise (20 mmol, 1 equiv). The resulting mixture was stirred at the same temperature for 2 h, resulting in the dissolution of the magnesium and the darkening of the solution. Then, the corresponding benzonitrile (20 mmol, 1 equiv) was added dropwise to the solution, and the mixture was allowed to stir at the same temperature for 16 h, resulting in the formation of a white salt. Thus, Me 3 SiCl (2.5 mL, 20 mmol, 1 equiv) was added dropwise with vigorous stirring after removing the heating, and the resulting mixture was stirred at room temperature for 16 h. A brown solid formed as a result, and the mixture was concentrated under reduced pressure and dissolved in benzene in order to filter off the salts. Benzene was then removed under reduced pressure, the resulting crude was dissolved in dry diethyl ether (10 mL), and the mixture was cooled to −78°C. Then, HCl (2 M in Et 2 O, 10 mL, 20 mmol, 1 equiv) was added, the resulting suspension was allowed to warm to room temperature over 30 min, and the solid was filtered, washed with diethyl ether, and dried under an IR lamp in order to afford the desired product. Preparation of Ketimines of Glycine Nitroanilides. 2-((Diphenylmethylene)amino)-N-(2-nitrophenyl)acetamide 1.
N-Boc glycine o-nitroanilide: 43 Boc-Gly-OH (1.38 g, 10 mmol, 1 equiv) and o-nitroaniline (1.38 g, 10 mmol, 1 equiv) were dissolved in dry pyridine (30 mL). The solution was cooled to −15°C, and phosphorus oxychloride (1 mL, 11 mmol, 1.1 equiv) was added dropwise with vigorous stirring. During addition, the reaction mixture was colored deeply red and slowly changed to brown. The reaction was complete after 30 min (monitored by TLC); afterward, it was N-Deprotection: To a solution of the previous N-Boc aminoamide (3 mmol) in CH 2 Cl 2 (12 mL) was added trifluoroacetic acid (4.5 mL) at 0°C. The mixture was stirred at room temperature for 30 min until full conversion (monitored by TLC). The solvents were evaporated, and the residue was coevaporated successively with a mixture of diethyl ether and pentane. Then it was dried in vacuo, and the resulting solid, obtained in a quantitative yield, was used in the next step without further purification. Iminoamide formation: To a suspension of the aminoamide trifluororacetate salt obtained in the previous step (927 mg, 3 mmol, 1 equiv) in CH 2 Cl 2 (11 mL) were added benzophenone imine (3 mmol, 1 equiv) and anhydrous MgSO 4 (903 mg, 7.5 mmol, 2.5 equiv). The reaction mixture was stirred at room temperature until consumption of the starting material. The mixture was then filtered to remove the salts and evaporated in vacuo. N-Boc glycine o-nitroanilide: It was prepared following the same protocol as in the case of iminoamide 1.
N-Deprotection and iminoamide formation: To a solution of the previous N-Boc aminoamide (3 mmol) in CH 2 Cl 2 (12 mL) was added trifluoroacetic acid (4.5 mL) at 0°C. The mixture was stirred at room temperature for 30 min until full conversion (monitored by TLC). The solvents were evaporated, and the residue was coevaporated successively with a mixture of diethyl ether and pentane. Then it was dried in vacuo, and the resulting solid, obtained in a quantitative yield, was used in the next step without further purification. The resulting aminoamide trifluoroacetate salt was neutralized by the addition of a saturated aqueous solution of NaHCO 3 , followed by successive extractions with dichloromethane (≈70% yield in the extraction). To a suspension of the resulting free 2-amino-N-(2-nitrophenyl)acetamide (585 mg, 3 mmol) in CH 2 Cl 2 (11 mL) were added bis(3,5-bis(trifluoromethyl)phenyl)methanimine hydrochloride (1.47 g, 3 mmol, 1 equiv) and anhydrous MgSO 4 (903 mg, 7.5 mmol, 2.5 equiv). The reaction mixture was stirred at room temperature until consumption of the starting material (monitored by 1 H NMR), then filtered to remove the salts, and evaporated in vacuo. 2-((Bis(4-(trifluoromethyl)phenyl)methylene)amino)-N-(2nitrophenyl)acetamide 3.
To a suspension of the free 2-amino-N-(2-nitrophenyl)acetamide, prepared as in the case of iminoamide 2, (585 mg, 3 mmol) in CH 2 Cl 2 (11 mL) were added bis(4-(trifluoro methyl)phenyl) methane iminium hydrochloride (1.06 g, 3 mmol, 1 equiv) and anhydrous MgSO 4 (903 mg, 7.5 mmol, 2.5 equiv). The reaction mixture was stirred at room temperature until consumption of the starting material, then filtered to remove the salts, and evaporated in vacuo. The crude was crushed in diethyl ether/hexane to afford a pure yellow solid. Yield: 75% (1. N-Boc glycine p-nitroanilide: It was prepared following the same procedure as in the case of iminoamide 1, but starting from pnitroaniline (1.38 g, 10 mmol) and was obtained as a white solid. Iminoamide formation: To a suspension of the aminoamide trifluororacetate salt obtained in the previous step (927 mg, 3 mmol, 1 equiv) in CH 2 Cl 2 (11 mL) were added benzophenone imine (0.5 mL, 3 mmol, 1 equiv) and anhydrous MgSO 4 (903 mg, 7.5 mmol, 2.5 equiv). The reaction mixture was stirred at room temperature until consumption of the starting material. The mixture was then filtered to remove the salts and evaporated in vacuo. N-Boc glycine 2,4-dinitroanilide: 43 Boc-Gly-OH (1.05 g, 6 mmol, 1.2 equiv) was dissolved in dry DMF (14 mL), and DIPEA (5.2 mL, 30 mmol, 6 equiv) was added at room temperature. Then, 2,4ninitroaniline (920 mg, 5 mmol, 1 equiv) was added, followed by HATU (2.09 g, 5.5 mmol, 1.1 equiv). The mixture was stirred at room temperature for 16 h. Then, a solution of EtOAc/H 2 O (1:1) was added to the reaction mixture, and it was extracted with EtOAc (3 × N-Deprotection: To a solution of the previously obtained N-Boc aminoamide (1.02 g, 3 mmol) in CH 2 Cl 2 (12 mL) was added trifluoroacetic acid (4.5 mL) at 0°C. The mixture was stirred at room temperature for 30 min until full conversion. The solvents were evaporated, and the residue was coevaporated successively with a mixture of diethyl ether and pentane. Then it was dried in vacuo, and the resulting solid, which was obtained in a quantitative yield, was used in the next step without further purification. Yield: quantitative. Iminoamide formation: To a suspension of the aminoamide trifluororacetate salt obtained in the previous step (1.06 mg, 3 mmol, 1 equiv) in CH 2 Cl 2 (11 mL) were added benzophenone imine (0.5 mL, 3 mmol, 1 equiv) and anhydrous MgSO 4 (903 mg, 7.5 mmol, 2.5 equiv). The reaction mixture was stirred at room temperature until consumption of the starting material (monitored by 1 H NMR). The mixture was then filtered to remove the salts and evaporated in vacuo. Preparation of N-Methyl Iminoamide 15.
In step 1, 44 N-methyl-2-nitroaniline (1.5 g, 10 mmol, 1 equiv) was dissolved in toluene (25 mL), and bromoacetyl bromide (1.0 mL, 12 mmol, 1 equiv) was added. The resulting solution was refluxed overnight (caution: HBr evolution), cooled, and concentrated in vacuo. The residue was purified through silica gel chromatography (eluting with hexane/EtOAc 70:30), affording the desired product as a yellow oil in 94% yield (2.27 g, 9.7 mmol). The spectroscopic data were coincident with those described in the literature. 45 46 to a solution of the bromide obtained in the previous step (546 mg, 2 mmol, 1 equiv) and DIPEA (0.35 mL, 2 mmol, 1 equiv) in dry acetonitrile (4 mL) under argon was added benzophenone imine (0.34 mL, 2 mmol, 1 equiv), and the resulting solution was refluxed for 5 h. The mixture was then cooled, dissolved in CH 2 Cl 2 (20 mL), and washed with aqueous 5% NaHCO 3 (20 mL). The water phase was extracted with CH 2 Cl 2 (2 × 15 mL), and the organic phases were combined and dried over MgSO 4 . The solvent was eliminated under reduced pressure, and the residue was purified through flash chromatography on silica gel (eluting with hexane/ EtOAc 60:40) to afford the desired product as a viscous orange oil in 70% yield (520 mg, 1. 39  Preparation of Benzophenone Imine of Glycine Methyl Ester 13. 47 To a suspension of glycine ester hydrochloride (377 mg, 3 mmol, 1 equiv) in DCM (6 mL) was added benzophenone imine (0.5 mL, 3 mmol, 1 equiv). Triethylamine (0.42 mL, 3 mmol, 1 equiv) was then added dropwise, and the reaction mixture was stirred at room temperature until consumption of the starting material (monitored by 1 H NMR). The mixture was then diluted with Et 2 O (6 mL), filtered, and washed with H 2 O (3 × 10 mL) and brine (3 × 10 mL). The combined organic layers were dried over MgSO 4 and evaporated to dryness. The crude was obtained with a quantitative yield (759 mg, 3 mmol) and was used without further purification. All of the spectroscopic data were coincident with those previously reported. 1  Preparation of α-Pyridyl and Phenyl Acetanilides 18−21. Synthesis of 2-(Pyridin-2-yl)acetanilides. 48 General Procedure.
To a suspension of 2-pyridylacetic acid hydrochloride (1.42 g, 6 mmol, 1.2 equiv) in dry Cl 2 CH 2 (12 mL) were added DIPEA (5.2 mL, 30 mmol, 6 equiv) and the corresponding aniline (5 mmol, 1 equiv), followed by HATU (2.09 g, 5.5 mmol, 1.1 equiv). The mixture was stirred at room temperature for 16 h. Then, a solution of Cl 2 CH 2 /H 2 O (1:1, 30 mL) was added to the reaction mixture, and it was extracted with Cl 2 CH 2 (3 × 50 mL), washed with brine (2 × 30 mL), dried over MgSO 4 , and evaporated under reduced pressure. The residue was purified by flash column chromatography on silica gel (eluting with hexane/EtOAc 80:20) to afford the desired product. Synthesis of N-(2-Nitrophenyl)-2-phenylacetamide 21. A solution of benzeneacetic acid (1.36 g, 10 mmol, 1.25 equiv) in Cl 2 CH 2 (10 mL) was cooled to −15°C, and SOCl 2 (101 mL, 15 mmol, 1.5 equiv) and DMF (2 drops) were added dropwise with vigorous stirring. After the mixture was stirred for 2 h at room temperature, a mixture of o-nitroaniline (8 mmol, 1 equiv) and K 2 CO 3 (1.5 g) in dry Cl 2 CH 2 (5 mL) was added, and the resulting suspension was stirred at room temperature for 16 h. The reaction mixture was then quenched with ice−water (100 mL) and extracted with EtOAc (4 × 60 mL). The organic phase was dried over MgSO 4 , and the solvent was evaporated in vacuo. General Procedure for the Asymmetric Aldol Reaction of Schiff Bases of Glycine Nitroanilide. The corresponding nitroanilide (0.2 mmol, 1 equiv) and the corresponding catalyst (0.02 mmol, 20 mol %) were dissolved in dry dichloromethane (0.5 mL) at the indicated temperature. To the mixture was added NaHCO 3 (0.02 mmol, 20 mol %) in one portion, followed by the corresponding freshly distilled aldehyde (previously washed with a saturated NaHCO 3 solution) (0.6 mmol, 3 equiv). The reaction mixture was stirred at the indicated temperature until consumption of the starting material (monitored by 1 H NMR). Then, MeOH (0.4 mL) was added, followed by NaBH 3 CN (32 mg, 0.5 mmol, 2.5 equiv) and AcOH (24 μL, 0.4 mmol, 2 equiv), and the mixture was stirred for 2 h (the reduction of the imine can be monitored by 1 H NMR). The solvents were evaporated under reduced pressure, and the residue was redissolved in dichloromethane and washed with a saturated NaHCO 3 solution (1 × 4 mL). The organic phase was dried over MgSO 4 and evaporated in vacuo. The crude was purified by flash column chromatography.
General Procedure for the Aldol Reaction of α-Pyridyl and Phenyl Acetanilides. To a solution of the corresponding acetanilide (0.2 mmol, 1 equiv) and Et 3 N (0.02 mmol, 20 mol %) in dry dichloromethane (0.5 mL) was added freshly distilled hydrocinnamaldehyde (80 μL, 0.6 mmol, 3 equiv) (previously washed with a saturated NaHCO 3 solution) at room temperature. The reaction mixture was stirred at the same temperature until consumption of the starting material (followed by 1 H NMR). To the reaction mixture was added pyridine/acetic anhydride (2:1, 0.4:0.2 mmol), and the resulting mixture was stirred at room temperature overnight. The mixture was then washed with 1 M HCl (1 × 4 mL) and saturated NaHCO 3 solution (1 × 4 mL). The organic phase was dried over MgSO 4 and evaporated in vacuo.
The general procedure was followed starting from N-(4-nitrophenyl)-2-(pyridin-2-yl)acetamide 19 (51 mg, 0.2 mmol). After 48 h at rt, the formation of the aldols as a 50:50 mixture of diastereoisomers was observed (50% conversion). The aldols were not isolated. See the 1 H NMR spectrum of the crude in the Supporting Information.
The general procedure was followed starting from N-phenyl-2-(pyridin-2-yl)acetamide 20 (42 mg, 0.2 mmol). After 48 h at rt, the formation of a 50:50 diastereomeric mixture of the aldols was observed (33% conversion). The aldols were not isolated. See the 1 H NMR spectrum of an aliquot in the Supporting Information. General Procedure for the Control Experiments. The corresponding compounds 13−15 (0.2 mmol, 1 equiv) and the corresponding catalyst (0.02 mmol, 20 mol %) were dissolved in dry dichloromethane (0.5 mL) at room temperature, and hydrocinnamaldehyde (0.6 mmol, 3 equiv) was added. When reaction was observed, the work-up described in the general procedure for the asymmetric aldol reactions was followed.
Stability of the Reaction Adducts.
The aldol adduct, previous to reduction, (0.2 mmol, 1 equiv) was dissolved in THF (5 mL), and 1 M HCl (0.68 mL, 0.68 mmol, 3.4 equiv) was added at 0°C. The mixture was stirred at the same temperature for 2 h. Then, after reaction completion (monitored by 1 H NMR), the solvent was evaporated under reduced pressure, and NaHCO 3 (sat) was added until pH 8−9. The mixture was extracted with Cl 2 CH 2 (3 × 10 mL), and brine (10 mL) was added to the aqueous phase, which was extracted again with Cl 2 CH 2 (3 × 5 mL). The combined organic layers were dried over MgSO 4 and evaporated under reduced pressure. The crude was used in the next step without further purification. Yield: 80% ( X-ray Crystallography. Crystals suitable for X-ray crystallography were obtained by crystallization of 1 from Cl 2 CH 2 , of 3 from Et 2 O/ CHCl 3 , and of 10 from CH 3 CN. Each sample was dissolved in the minimum amount of the indicated solvent at rt and was allowed to crystallize slowly at the same temperature. Intensity data were collected on an Agilent Technologies Super-Nova diffractometer, which was equipped with monochromated Cu Kα radiation (λ = 1.54184 Å) and Atlas CCD detector. Measurements were carried out at 150.01(10) K with the help of an Oxford Cryostream 700 PLUS temperature device. Data frames were processed (united cell determination, analytical absorption correction with face indexing, intensity data integration, and correction for Lorentz and polarization effects) using the Crysalis software package. 50 The structure was solved using SHELXT 51 and refined by full-matrix least-squares with SHELXL-97. 51,52 Final geometrical calculations were carried out with Mercury 53 and PLATON 54 as integrated in WinGX. 55 Complete structural data have been deposited with the Cambridge Crystallographic Data Centre. 29,31 ■ ASSOCIATED CONTENT
Catalyst screening data; X-ray diffraction data for 1, 3, and 10; computational data for the E and Z enolates of ketimine 3; spectroscopic data for all new compounds including 1 H and 13 C NMR spectra; HPLC chromatograms of all the aldol adducts (PDF)